1. Field of the Invention
This invention generally relates to rivets and particularly relates to rivets for attaching brake linings to brake shoes.
2. Description of the Related Art
A brake shoe assembly for drum brakes typically comprises a brake shoe, which has an inner frame having a curved backing plate on the periphery of the frame, and a curved lining of frictional material attached to the outer surface of the backing plate of the shoe. The lining is formed to have an inner radius equal to the outer radius of the backing plate. The friction lining is usually attached by adhesively bonding the lining to the backing plate or by riveting the lining to the backing plate, which is shown generally in FIG. 4. During use, the lining contacts the inner surface of a drum (not shown), the lining acting as a sacrificial wear element. The brake shoe may be rebuilt after use by removing the remaining lining from the shoe and attaching a new lining.
Riveting is often used for high load applications, such as brake shoes for truck brakes, and it is critical to the proper performance of the truck brake that the lining maintains its position relative to the brake shoe during the useful life of the lining. If the lining is not tightly secured to the brake shoe, the lining can shift when the brakes of the truck are applied. This shifting of the lining can cause excessively rapid wear of the lining and/or a complete malfunction of the truck brakes, causing danger to the truck, the operator, and those in the path of the truck.
The current rivet design is based on the 150 Degree Countersunk Head Semi-Tubular Rivet, as specified in the American Society of Mechanical Engineers (ASME) standard B18.7 1972 (R1980), which is shown in FIG. 1. The standard defines a rivet with a cylindrical body having a head at one end and a hole in the end of the body opposite the head. The reference depth K of the hole is approximately equal to the diameter B of the cylindrical body. The inner end of the hole has a conical shape, the included angle being from a maximum of 120 degrees to 180 degrees, which forms a flat inner end.
In the past, a different rivet length was used to secure each lining thickness to the brake shoe so that the depth of the hole in the rivet could be maintained at approximately equal to one body diameter. When the semi-tubular rivet is installed, the rivet is pushed through a hole in the lining and then through the backing plate of the brake shoe. Pressure is applied to the rivet head to keep it tight in the shoe assembly while an installation anvil strikes the exposed tubular end of the rivet, making the walls of the tube roll outwardly and toward the head of the rivet. This shortens the body of the rivet and creates a clamping force between the lining and the shoe. If the inner end, or base, of the hole in the rivet extends beyond the inner surface of the backing plate when the rolling begins, the sidewall of the hole will stop rolling before a clamp force has been created, resulting in loose brake shoe linings.
To overcome this concern, truck brake manufactures have modified the standard rivet design to increase the depth of the hole in the rivet, enabling a rivet of one length to be used to secure linings of various thickness to the brake shoes. FIGS. 2 shows an example of this type of rivet. Rivet 11 has a body 13, one end of the body having a planar head (not shown), the other end of the body having a cylindrical sidewall 15 enclosing a hole 17. Hole 17 has a cylindrical surface 19 that is generally parallel to the outer surface of sidewall 15, cylindrical surface 19 terminating in a base 21. Though shown as being curved and having a large radius, base may be conical, as shown in ASME B18.7. The depth of hole 17 exceeds the diameter of the outer surface of sidewall 15, and the intersection of base 21 with cylindrical surface 19 is at a sharp comer 23.
However, increasing the depth of the rivet""s hole has a practical limit. As the depth of the hole progressively increased beyond the diameter of the body, the rivet sidewalls begin to collapse, instead of rolling outward and downward, and/or fracture when the installation anvil applies pressure to the tubular end of the rivet. This is illustrated in FIG. 3, which shows rivet 11 after installation. The right portion of the figure shows sidewall 15 being rolled over, but crack 25 has formed at the intersection with cylindrical surface 19 and base 21. Likewise, the left portion of the figure shows sidewall 15 collapsed, forming fold 27 near the intersection with base 21. When sidewall 15 collapses or develops cracks, the required clamp force is not consistently achieved in all rivets 11 in a truck brake shoe assembly, and loose brake linings can result.
Thus, there is a need for an improved rivet for joining brake linings to brake shoes, in which the depth of the hole in the rivet may be greater than the diameter of the body without collapse or tearing of the sidewalls of the rivet during installation.
An improved rivet is provided for joining a friction lining to a brake shoe. The rivet has a cylindrical body with first and second ends. A head is located on the first end of the body, and a hole is formed in the second end of the body. The hole terminates at a base within the body, the hole having a cylindrical surface that is concentric with the body, the base and the cylindrical surface being connected by a circumferential transition region. The depth of the hole, as measured from the second end of the body, is greater than the outer diameter of the body. In one embodiment, the transition region is a curved surface having a radius greater than one third of the radius of the hole. In another embodiment, the transition region has a conical surface that lies at an angle relative to the cylindrical surface and the base, the conical surface being connected to the cylindrical surface and the base with curved surfaces.